I owned the 811 with the three tubes for awhile but everybody I know who has
one, says you need to be careful not to overload, or over drive it, and not
because the tubes can't take it.  It is not a heavy duty amp but works well.
You can replace the 811A tubes with 572B tubes, which not only cost more,
but they will run more power.  This is where you can easily get into
trouble.  The 811A tubes are dandies.  The 811H is a good amp, too, as long
as you are careful tuning it and don't get carried away driving it too hard.
I don't care what anybody says.  I ran an old Heath Kit Warrior amp running
four of the 811A tubes for many years.  A friend helped solid-state the
power supply and my idle voltage jumped up by 200 volts.  This means my
driving voltage jumped up, too, of course, and I was getting 700 watts out
of that old 90 pound amplifier with 100 watts of drive.  I ran the tubes for
literally thousands of hours before my first replacement and even then, I
had one tube with broken off tiny fragments of the plate rolling around
inside the tube before I got enough to buy four new tubes.  My output was
down to 450 watts but those old 811A tubes were still cooking.  Unlike the
572B tubes which won't take as much punishment and over driving, the 811A
tube really is a work horse.  Of course, this was back in the days before
China and Russia made all the tubes so I have no idea how good those tubes
are.  I have heard that many of the American made amps, run the Russian
tubes and I have had friends who have purchased, and run, the Chinese tubes.
Unless you want to run 1.5K output, you could not pay me to run a tube
amplifier any more.  I would go solid-state any day of the week.  Yes, it
cost twice as much and yes, the power supply is a beast and cost a fortune,
too.  Take my word for it, tubes less amplifiers is the best way to go for
500 to 600 watts output.  Besides, if you ever fry the parasitic chokes on
top of those 811A tubes, it will take you a year to get the stink out of
your house.  I know.  I did it twice over about 10 years of use.  My nose no
longer works and I swear that's the reason.

Phil.
K0NX